Mobile radio networks are known to the specialist from various standards such as GSM etc. When transmitting from one mobile radio user to another mobile radio user via a mobile radio network, data (voice data or other data) in accordance with predetermined parameters (relevant bit rate, coding etc., known as codecs) is transmitted in coded form on the air interface of the mobile radio network. In a Transcoder Rate Adaptation Unit (TRAU) of a base station, the data is reformatted (transcoded) (as a rule the common fixed-network PCM format etc.), is transmitted further circuit-switched or packet-switched, recoded again (transcoded) before transmission to the second user via a second air interface with the parameters known to the second user (a codec mode known to this user) into a format suitable for transmission via the air interface. The repeated re-coding impairs the voice quality. If two communicating users within a mobile radio network know of at least one similar transmission format (or defining parameters), i.e. at least one same codec, data from one telecommunication user to another telecommunication user can be transmitted without transcoding. This method of transmission, also known as tandem free operation (operation without dual tandem transcoding) increases the quality and reduces the required bandwidth, because a smaller bandwidth can be used for transcoder-free transmission than for transcoded transmission.
For a multi-rate codec AMR-WB (which divides the radio bandwidth, according to radio conditions, into wanted and protective signals and enables eight out of nine different modes) the (at least) two users exchange the modes that are to be supported for communication between them (supported codec set) and which are actually to be used (active codec set). The modes (maximum of four in GSM) on which both users agree is then determined by means of a specified algorithm. Generally the voice quality is also taken into account during this. For example, there is a rule which states that at least one of the bottom three (for AMR half-rate) or five (for AMR full-rate) modes must be present for a TFO communication.